Wireless communications devices, for instance, mobile phones have become popular remarkably in recent years with their functions and services improved increasingly. Explanation will be made on a mobile phone as an example. There are various systems for mobile phones, for instance, GSM (Global System for Mobile Communications) and DCS1800 (Digital Cellular System 1800) systems widely used mostly in Europe, a PCS (Personal Communications Services) system used in the U.S., and a PDC (Personal Digital Cellular) system used in Japan. According to recent rapid expansion of mobile phones, however, a frequency band allocated to each system cannot allow all users to use their mobile phones in major cities in advanced countries, resulting in difficulty in connection and thus causing such a problem that mobile phones are sometimes disconnected during communication. Thus, proposal was made to permit users to utilize a plurality of systems, thereby increasing substantially usable frequency, and further to expand serviceable territories and to effectively use communications infrastructure of each system.
As a mobile phone having this new system, dual-band mobile phones (see Japanese Patent 2,983,016), triple-band mobile phones, etc. are proposed. While a usual mobile phone comprises only one transmitting/receiving system, the dual-band mobile phone comprises two transmitting/receiving systems, and the triple-band mobile phone comprises three transmitting/receiving systems. With these structures, users can choose and utilize available transmitting/receiving systems among a plurality of systems.
Such a mobile phone comprises a high-frequency switch module comprising a branching circuit (diplexer) for directing a received signal to either one of a low-frequency system and a high-frequency system depending on a frequency band of each system, and a high-frequency switch circuit for switching signal paths of a reception line and a transmission line. Usable as a means for separating reception and transmission in place of the high-frequency switch for switching lines is a branching circuit (duplexer) comprising a bandpass filter utilizing frequency difference between reception frequency and transmission frequency. For instance, Japanese Patent Laid-Open No. 8-321738 discloses a duplexer comprising a combination of a first bandpass filter and a first phase shifter with a second bandpass filter and a second phase shifter for branching signals of different frequencies, without comprising the high-frequency switch. However, Japanese Patent Laid-Open No. 8-321738 fails to refer to a combination of such duplexer with the high-frequency switch to provide a high-frequency switch circuit adapted to larger numbers of frequency bands, and the formation of these circuits to a module to provide small and lightweight wireless communications devices.